STRATIGRAPHY AND PROVENANCE OF THE POUL CREEK FORMATION IN THE CHUGACH-ST. ELIAS MOUNTAINS, SOUTHEAST ALASKA: INSIGHTS ON EOCENE-MIOCENE SEDIMENTATION IN THE GULF OF ALASKA

New stratigraphic, sedimentologic, and U-Pb detrital-zircon geochronologic data have been analyzed from the Poul Creek Formation (PCF) in the Chugach-St. Elias Mountains of southeastern Alaska. These sedimentary strata provide a record of sedimentation during Eocene – Miocene time as the Yakutat microplate was transported and subducted beneath the Alaskan continental margin.

The stratigraphy and sedimentology of the PCF varies laterally from thick successions of mudstones in the east near Cape Yakataga to coarser-grained sedimentary strata in the western part of the study area near Katalla. The eastern PCF contains organic-rich, occasionally glauconitic shales and sandstones interbedded with fine-grained, thin-bedded turbidites. These strata are interpreted as being deposited in an upper slope to base-of-slope environment. The base of the western PCF is characterized by ubiquitous fine- to medium-grained, low-angle cross-stratified sandstones. Abundant hummocky cross-stratification, bioturbated siltstones, fine-grained sandstones, and shales are found overlying the basal sandstones of the western PCF. These lateral facies changes infer more shallow sedimentation to the west within outer-shelf to shelf-break settings. The limited extent of this facies indicates a locally restricted, well-developed continental shelf compared to the rest of the coastal margin at this time. The presence of thick pillow-basalts interbedded with the upper PCF indicates deposition proximal to an arc system in the early Miocene. Oligocene to early Miocene plutons adjacent to the Yakutat microplate along the Gulf of Alaska are likely the source of the abundant volcanic rocks within the PCF.

Provenance data is consistent with sourcing from Cenozoic to Mesozoic plutons common along the northwestern margin of the Cordillera. Peak detrital zircon age populations in the PCF occur between 50-95 Ma and 120-160 Ma. Similar provenance of the PCF with that of both the underlying and overlying stratigraphy suggests abundant sediment recycling within the Chugach-St. Elias fold and thrust belt.